fall for a 1:60 rainwater soakaway

Hi can someone help,
I need to know how to work out the fall, I am putting in a rainwater soak away 5m from the annexe. What would the fall be, I have looked on various web sites and worked it out to be around 100mm is that right and if so how do I work this out. I also need to work out foul water fall for other drains is this a different calculation.

Thanks in advance.
Cliff

Over a 5 metre run

50mm = 1:100
63mm = 1:80
85mm = 1:60
125mm = 1:40


All the regs are available here

http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_H_2010.pdf

That's really helpful thanks very much
Cliff

Take a length of timber a metre or so long. Put a screw in one end to the depth of your fall. Use it with a spirit level on top. When the level shows level the fall should be correct.

I have printed out the document and will read through it, but one question you may know. does it stipulate how deep I need to go for the trench?

Thanks
Cliff

Document H1 - 2.27 / 2.28

cliff1010 said:

I have printed out the document and will read through it, but one question you may know. does it stipulate how deep I need to go for the trench?

Thanks
Cliff

Click to expand...

You would typically dig the soakaway first. This will reveal to you your soil conditions as you descend. Your inlet pipe will need to be a minimum of say 600mm down from the surface (even more if the ground rises away from the building) so that you have decent soil coverage over the pipe and that you are into decent draining ground.

Your pipe will then need to rise steadily at a rate that is compatible with the furthest gully away from the soakaway.

We set the furthest gully away first and look at what is the maximum achievable fall we can get whilst looking at the minimum possible depth we can put the pipe at the low end. We fiddle about with the trenches until we're happy with the falls.

25mm every metre (1:40) is ideal. however, with careful digging and packing of the pipes, the system will work with much less fall, i.e. 1:80. Lesser fall means a greater attention must be paid when setting the pipes in gravel i.e. no dips, no humps and nice straight lines and uncluttered junctions etc.

Really helpful thanks that answers my questions.

Cliff